1. Technical Field
The present invention relates to a wire material for manufacturing a piston ring, used for a piston of an internal combustion engine, and more particularly, to a wire material for manufacturing a combination oil ring.
2. Background Art
Piston rings generally include a pressure ring and an oil ring. Examples of such piston ring are: those comprising two particular kinds of rings (one pressure ring and one oil ring); and in a case of a top ring for diesel engine, those comprising a coil expander and the like formed of a heat resistant material, for example, a steel material. Further, in an oil ring, the coil expander is used when the oil ring is arranged into a known two-piece oil ring, etc.
In these piston rings, the oil ring, for example, requires a larger tangential force than the other pressure rings to satisfy its functions, that is, an oil scrapping-off function and an oil controlling function. For this purpose, a surface pressure is increased by narrowing a contact width and reducing a contact area, thereby a sealing property and an oil scraping-off property are enhanced. However, by simply increasing the tangential force, it will be a cause of an increase of a friction. Accordingly, when the tangential force is reduced in order to reduce friction, the functions such as the oil sealing property and the oil controlling function cannot be sufficiently obtained, particularly in a high revolution region.
Under these circumstances, a “variable tangential force ring”, which has a low tangential force in a low revolution region and has a high tangential force in a high revolution region, is under study. As a known background art document, Patent Document 1 discloses a technology of an oil ring using a coil expander formed of Ni—Ti based shape memory alloy, in which the coil expander is treated such that it is in a contracted state at low temperature and in an expanded state at high temperature.
As described above, the force of the coil expander for pressing an oil ring outward in the radial direction thereof can be changed depending on temperature by forming the coil expander of the shape memory alloy. Accordingly, it is effective for reducing the friction, as well as improving starting property of an engine. However, the modulus of transverse elasticity of a shape memory alloy material is about 6000 to 9000 MPa when it is in a low temperature phase (martensite phase) and about 20000 MPa in a high temperature phase (austenite phase) in a Ni—Ti based binary system. Since these numerical values are only about ¼ compared to that of a coil expander composed of an ordinarily used steel wire material, a wire material composed of shape memory alloy must be four times as thick as the steel wire material to obtain the same tangential force. In contrast, since the width of recent oil rings tend to be reduced to enhance a following property, it is difficult to put a coil expander formed of shape memory alloy to practical use due to restriction in size.
To overcome these problems, there is proposed a coil expander having a rectangular cross-sectional shape. However, in the coil expander having the rectangular cross-sectional shape, the cross-sectional shape thereof may be deformed into a concave shape, for example, on the outer peripheral surface thereof due to stress such as tangential force, compression, etc. that is caused when the coil expander is bent into a coil shape.
As described above, when the coil expander is deformed into the concave shape, the coil expander locally slides strongly against the inner peripheral groove of an oil ring in which it is disposed. Therefore, a problem arises in that abnormal wear occurs and strength reduction as the result. To prevent the local sliding due to deformation, there is employed a method of using a wire material, whose thickness in the radial direction of a coil expander is made larger, making such wire material into a coil, and then, obtaining sliding surface by subjecting the outer peripheral surface of the wire material to centerless machining. However, the method is not preferable because the material is wasted by the machining and the number of steps is increased by cutting the wire material.
Further, although it is not related to the coil expander, as a technology related to the cross-sectional shape of a spring, Patent Document 2 discloses the cross-sectional shape of a wire material of a coil spring used for a valve gear of an internal combustion engine. In Patent Document 2, a wire material of an inner valve spring is formed into an asymmetric cross-sectional shape, thereby providing the respective portions thereof with different characteristics. Although the valve spring and the coil expander are both arranged into similar spring shape, since they have a thoroughly different purpose of use and function, the technology of the coil spring cannot be applied to the coil expander.
Further, Patent Document 3 discloses a seal ring having a cross-sectional shape composed of curved surfaces having a different curvature. As in the case of the above-described coil expander, this is to cope with the change of the cross-sectional shape caused when a wire material is formed into a ring shape. However, since the seal ring and the coil expander have a quite different purpose of use and function, applied curvature when forming into a coil shape is also quite different, and a different characteristic is required to them. Therefore, the technology disclosed in Patent Document 3 cannot be applied to the coil expander.    Patent Document 1: Japanese Utility Model Application Publication No. 3-41078    Patent document 2: Japanese Utility Model Application Laid-Open No. 63-92011    Patent document 3: Japanese Utility Model Application Laid-Open No. 52-1933